Hi guys,
          Could anyone say what are (if any) advantages to using continuous time sigma delta over discrete time (Switched Cap) Sigma delta modulator?
           
         Thanks for your time
         shaf
         

raja.cedt wrote on Dec 2^nd, 2010, 7:32pm:


The first point is clear of course. You have implicit antialiasing which helps a lot in some cases.

The second point is a bit obscure to me. Why should the loop pole/zero locations have anything to do with the input impedance. At any rate, the fixed absolute location of poles/zeros in the NTF is one of the major DISADVANTAGES of using a continuous-time modulator, especially since you are likely to want to use a modulator for different frequency ranges, e.g. in case you want to make it for a wireless application, then for different standards.

A discrete-time solution in this case is more elegant as the NTF remains independent of the operating clock frequency, allowing the same modulator to be scaled and used for different applications, provided that the settling requirements are always fulfilled.

And there lies the catch. Since discrete-time modulators require very good settling and are typically realized in form of switched-capacitor filters, they consume much more power for the same operating frequency, and are also not useful once you start clocking at really high rates.....

There are other differences too of course, but these seem to be the most important ones.

Vivek

indeed, it is true that the amp in a CT DSM has to settle too. However, let's put it this way - comparing a DT DSM to a CT DSM is like comparing apples to oranges as the two have very different features. let me try to answer your first question though:

A DT modulator is usually restricted by its step response, and more specifically by the slewing in its amplifier. In a CT modulator, slewing is not allowed, which means that the amplifier technically is always operating in a "small-signal" mode, and the step response is not the limiting feature by design, provided that the design was done correctly. A couple other points help too, like the fact that you will likely have (will need) a multi-bit DAC in a CT DSM to get away from the problem of jitter. So you will intrinsically have a smaller DAC step in most cases.

The other side concerns linearity. DT DSMs typically are made for relatively high linearity => the settling of amps becomes more critical. Since the CT DSMs are usually not running in a slew-limited mode, the settling is not going to determine the linearity. Of course, the comparison is again flawed since you will anyway not be designing (and will also not achieve) high THD with a CT DSM. These aim for more modest THD.

Hope that makes sense.

Vivek